Plastic closures, typically injection molded from polyethylene, have been used for many years in combination with open head shipping containers. Such closures typically provide a "plug fit" with the container; i.e. the closure structure defines a circular, inverted U-shaped channel which opens downwardly to receive the top edge of the container therein. When applied to a container, the inside wall of the channel lies against the inside surface of the container. See, for example, U.S. Pat. No. 4,711,364 to Ilija Letica.
In closures intended for heavy duty usage, this inside wall, called a "wiper", can extend quite far into the container. In such designs, it is desirable to form another vertical circular wall, herein called a "riser wall", inboard of the wiper so that the center panel of the closure is placed closer to the top of the container wall; this effectively increases the container interior volume. In this case an upwardly opening U-shaped channel is formed between the wiper wall and the riser wall.
In prior art closures, bridge ribs have been formed integrally with and extending radially between the wiper wall and the riser wall to provide very high radial rigidity in the structure of the closure. It has been found, however, that these integral and radial bridge ribs act as stress concentrators in the event of extremely high side loads that are typically created when a container is inadvertently dropped on its side in a filled and sealed condition. Such extreme loads occasionally cause fracturing of the radial bridge ribs and/or the center panel of the closure structure, thereby creating a loss of structural integrity in the closure and possible spoilage or contamination of the contents within the container.